Electric amplifier with negative feedback



Filed Feb. '7. 1947 2 SHEETS-SHEET 1 p 23, 5 A. H. ROCHE 2,611,833

ELECTRIC AMPLIFIER WITH NEGATIVE FEEDBACK Inientor By A ltorze Sept. 23, 1952 A. H. ROCHE 2,611,333

ELECTRIC AMPLIFIER WITH NEGATIVE FEEDBACK Filed Feb. 7. 1947 2 SHEETS-SHEET 2 I M%%peztor y A Home y Patented Sept. 23,1952

I J 2,611,833 ELECTRICIAMPLIFIER WITH NEGATlVE FEEDBACK Alleman Holly Roche, London, England, assignor to International Standard; Electric Corporation, New York, .N. Y.,.a corporation of Delaware Application Feb ruary 7, 1947, Serial No. 727,208- In'Great- Britain April 30 I946 1 The present invention relates to electric' wave amplifiers having negative feed-back; and is concerned with arrangementsforcompensating for the gain variations-resulting from-the ageing of the valves-of the amplifier.

During the life of an amplifying valve, the anode current tends continually to decrease and at the sametimethe mutual 'conductan'ceor slope of the valve also decreases, so that the'gain' of the amplifier decreases. When an amplifier is provided with stabilising negative feed-'back this gainreduction is to some extent compensated by thene'gative feed-back, but the degree of compensation depends on the frequency and it" is known that generally the compensation is practically complete at the'extremeends of the frequency band for which the amplifier is designed,

but is only partial for frequencies near the middle of the band. In order to'minimise the iniportance of this efiect, it has previously been proposed slightly to modify thegain-irequency characteristic of the amplifier so that when the valves are new, thegain at frequencies near the middle of the band is slightly too .high, and so that at the end of the life of the valves the gain atthese frequencies is approximately the same amount too low. This method hasthe objectionthat the gain characteristic of the amplifier is correct only for a comparatively short period at the middle of the life of the valves;

It is'the principal object of the present invention to provide an arrangement by means of which the gain-frequency characteristic of the amplifier is continuously corrected during the ageing period of the valves,.sothat it has the prescribed form substantially for the whole of their lives.

This object is achieved according to theinvention by providing an amplifier having a forward amplifying path and a negative feed-back path, comprising means under the control of the anode current of one or more of the valves in the forward amplifying path whereby the gain of the amplifier at certain frequencies is adjusted by'an amount depending on the magnitude of the said anode current. In a, di-iferent form of the invention',,the means under control-of the anode current is provided for maintaining substantially unaltered the gain-frequency characteristic of the amplifier in spite of the ageing of the'valves.

The invention will be described with reference to the accompanying. drawings, in which:

Fig. 1 shows gain-frequencychraracteristics'of an amplifier with negative feed-back in order to indicate the effects of ageing of the valves;

1 Claim. (01. 179-171;)

270% and not so good at freque'ncies 'where'the on Fig. 2"shows a simplifiedschematic:circuitdiagram of anamplifierhaving meansior correct.- ing the gain-frequency characteristic according to the invention; I

Figs. 3: and 4 show detailsof; networkslwhich maybe usedinthecircuit: of Fig. 2;, and Fig: 5

shows a detailed schematic circuit; diagram of an amplifier incorporating: the features of.the.-in'- vention. v11

Ashas already beenmentioned, the: gain variations caused-by the ageing of the valves in an aniplifier areonly partly stabilised by negative feed:- back. It is well known that stabilisation is practically complete at frequencies forwhich. the phase shift round the feed-back loop is 90 or phase shift is 180. Thus, referring to Fig. 1,-F1 and F2 are the maximum and minimum frequencies of the band to be transmitted, and curve! gives an example of thespecified gain frequency characteristic of a negative feed back amplifier Y which it has whenthe valvesare-new. Towards the endof the liieof thevalves it may be'found thatthe characteristic has changed to the dotted line 2. At frequencies F1 and F2 the loop phase shift is usually inthe neighbourhood of 90 or 270, so as already explained; the gain does not change appreciably at these frequencies. At the centre of the frequencyband, however, the loop phase shift is near 180" and the gain is-reduced in the manner indicated by curve 2 at the en of'the life of the valves. 7 According to the previous proposal already referred to, the gain of the. amplifier is modified so that when the valves are new, the characteristic produced lies above the curve I byanamount substantially equalto half the difference between the curves l and 2, so that, while the departure from they desired characteristic does not exceed half the maximum difference between the two curves, the specified characteristic is only fobtained for a short period of time.

.. According to the presentv invention, however, the gain characteristic is continually corrected duringthe ageing period of the valves sothatat all times it coincides substantially with the specified curve. Fig. 2 shows one arrangement by which this continuous correction is effected. In Fig. 2, the block 3 represents the forward amplifying path of the amplifier. It may consist of any number of stages of amplification. arranged in any known way. Only the control grid 4 of the first valve and the anodev 5 of the last valve are indicated; and 6 represents one or more of the. valve cathodes which are usually connected in parallel for controlling the thermistor.

to the negative terminal of the high tension source through bias networks or the like (not shown).

The anode 5 is connected to the positive high tension terminal i through an appropriate load impedance or network 8: The negative high ten sion terminal 9 will usually be grounded, either directly, as shown, or through a large by-pass Condenser. Amplified signals are taken from the anode 5 through a blocking condenser it and through an output transformer i l, the secondary winding of which is connected to output terminals l2 and I3.

The primary winding of the transformer H is connected to terminal 9 through two impedance networks I l and I5, connected in parallel with a resistance 16, and a feed-back connection H is taken from the junction point of these two impedances to the control grid 4 through the secondary winding l8 of the input transformer l9,

6 and the terminal 9,. a by-pass condenser 25' being provided to shunt the heating coil. A'suitable indirectly'heated thermistor is described in British patent specifications Nos. 541,922, 555,563 or 557,559. I

It will'be evident that the attenuation of the feed-back path depends on the values of the impedances M, and I5 shunted by 22. The resistance 22 depends on the magnitude of the current which flows from the cathode 6 through the heating coil 24 of the thermistor 23, and the current is equal to thesum of the anode currents of all the valves of the forward amplifying path 3 whose cathodes are included in 6. If, as is commonly the case, the thermistor has a negative temperature coefiicient of resistance, the resistance 22 will be relatively small when the valves are new and the combined anode current is large. The networks M and [5 will be designed according to known principles so that in this condition the complete amplifier has the specified gain-frequency characteristic, such as curve I, Fig. 1.

As already explained, when the valves are old, the amplifier tends to have a characteristic like curve 2, Fig. 1. However, in this condition, the combined anode current is smaller than before, so that the resistance 22 is increased. The networks [4 and I5 will therefore be so designed that slightly reduced feed-back is produced at the middle of the frequency band by the amplifier but not at the ends, s that the curve 2 is corrected to coincide with the curve I.

In order to obtain this sort of correction, network I should have the characteristics of a parallel resonant circuit.

It will be understood that the heating coil 24 of the thermistor 23 may be connected in series with only one of the cathodes of the valves in the forward amplifying path 3, or in series with any number of them. In general, of course, it is preferable to include all the cathodes, since thereby the effects of ageing of all the valves will be used For all cases, of course, the heating coil should be shunted by a by-pass condenser 25 in order to keep the signal currents out of the thermistor.

Fig. 3 shows an example of a suitable configuration for the'network I4 of Fig. 2, which gives a characteristic of the type shown by curve I of resistance 33, and inductance 34 shunted by resistance 35,,are introduced to assist in the shaping .of the characteristics of the feed-back loop as explained, for example, in British patent specifications Nos. 499,315 and 514,567. The condenser 35 represents the unavoidable capacity acting across-the feed-back path, which must be taken into account, and does not represent an actual element which has to be provided.

The network 15 is Fig. 2 may be as shown in Fig. 4. The requirements are that the impedance of the network shall not change when the thermistor resistance changes, at frequencies where the loop phase-change of the amplifier is 90 or 270, and shall change by the required amount at frequencies where the loop phase change is 180. The maximum change will be small in most prac tical cases. The network of Fig. 4 accordingly comprises between the terminals 31 and 38 two meshes consisting respectively of a condenser 39 shunted by a resistance 40-and an inductance 4i shunted by a series combination of a condenser 42'and a resistance 43. A condenser 45 connected betweenthe terminals 3'! and 38assists in shaping the feed-back loop characteristic at high frequencies.

This network includes the parallel resonant combinationreferred to above, consisting ofthe elements 4! and. 42. The network elements should be chosen so that its impedance at frequencies F1 and F2 is small compared with the resistance 22 of the thermistor 23, so that at these frequencies, the thermistor does not appreciably affect the feed-back. If the elements 4|, 42 resonate at a frequency near the middle of the band, the impedance of the network will increase so that the thermistor then has the necessary effect in reducing the feed-back as the valves age. It willbe evident that it is necessary to determine the relation between the-anode current of the valves and the corresponding mutual conductance under the specified, operating condition of the valves in order that the values of the elements of Figs. 3 and 4, may be determined. When this relation and the'requirements for the amplifier are given, those skilled in the art will have no difiiculty in designing the networks whose configuration has been given.

Various other configurations are possible for thenetworks l4 and I5; and the resistance element-22 of the thermistor might be arranged in series or in shunt with part only of the network l5.

Similar principles may be applied to amplifiers in which the feed-back path takes other forms. For example, when the feed-back is obtained by means of inputand/or output bridges, the thermistor may be connected in such manner as to operate on one of the arms of one of the bridges or one diagonal thereof.

It is also possible to employ several thermistors, the resistance elements of which are connected in various parts of the feed-back path, the heat ing coils being connected in series'with one or more of the valves in the forward amplifying path, and other kinds of non-linear resistance elements such as voltage-dependent resistances of the thermistor could be included in orie terstage coupling networks the forwagainfof the forwardpath in th desired ithrougho ut the frequencyband; as th' conductance of-the valves decreases gageing p'e'riod; Alternativelythe thermistorcould and. rectifiers could be empldyed in conjunction with ithe'thermistors. v

There are also other waysiin-zwhich thegain frequency. oharacteristi of, the amplifier could be corrected under the control of the valve anode currents. For example, the resistance' eler'nent &

operate on a local feed-back path' -provided for one of the valves in the forwa damplifying path,..so

1 5 amsuitably Vary f f iiu'ency selecli llitybf of the amplifier. Any combination of these various methods could clearly be employed.

A practical example of an amplifier employing an arrangement according to the invention is shown in Fig. 5. This is a three stage amplifier intended to be installed at an unattended station and to be supplied with operating power from a central or control station over the conductors which carry the signal currents to be amplified. Such an amplifier is suitable, for example, for use as a submerged repeater on a long submarine cable.

Fig. 5, as far as the features of the invention are concerned, is similar to Fig. 2, and similar elements in both figures will be given the same designation numbers.

The circuits connected to the input and output of the amplifier are shown as co-axial lines 46 and 41, the outer conductors of which are connected to ground. In order to supply operating power for the amplifier, the positive terminal of a direct current source (not shown) is connected to the inner conductor of the line 46 at a preceding station, the negative terminal being grounded. The inner conductor of the line 46 is connected through an inductance 48, through the cathode heaters 49, 56 and 5| (shown separately. from the three valves for clearness), in series with a resistance 52 and through a second inductance 53 to the inner conductor of the line 41, and thence to other unattended amplifiers (if any) and finally to ground. The current is adjusted to the value required for the cathode heaters, and the resistance 52 is chosen so that the total drop of potential across the heaters and this resistance is equal to the high tension voltage specified for the amplifier. If high resistance heaters are used, the resistance 52 may not be required.

The positive high tension terminal I for the amplifier is accordingly the junction point of elements 52 and 53, and the negative terminal 9 is the junction point of elements 48 and 49. In this case terminal 9 cannot be directly grounded, but is effectively connected to ground by a by-pass condenser 54 which also forms a low pass filter with the inductance 48 for separating the signal and power currents. Likewise a by-pass conthe inductance 53,

high resistances 14 and 15.

denser 55 forms a similanlowfiass filt similar pentodes arranged in a conventional Way} The three cathodes ar e conne'ctedi -commonconductor"59 through network 69,

"Stand 62. The cornnion conductor59is connected to terminal 9through thethermistor heating 'c'oil 24 shunted b'y 'the by pass'condenser' 25. The suppressor'grids of the valves'iare'di'retly eenne cted to the respective cathodes and the screen "grids are connected 'to terminal T through corresponding resistancestd' 64 -and 65 and-to the common conductor 59 through by-pass condens'er's 136, '61 arid BB. Th'e'anodes o f"'the"" a1 is connected through the primary winding of the output transformer l l and through the primary winding of the feed-back circuit coupling transformer H to terminal I. the valves 51 and 58 are connected to the anodes of the respective preceding valves 56 and, 51 through coupling condensers I2 and 13, and to the common conductor 59 through the usual The control grid of the valve 56 is connected through the secondary winding [8 of the input transformer I9 to the feed-back conductor [1. The secondary winding of the feed-back circuit coupling transformer H is connected across the networks l4 and 15 which with the thermistor 23 are arranged as in Fig. 2. The only difference in this part of the circuit from Fig. 2 is the interposition of the transformer II instead of the resistance I6 which does not affect the principle of operation of the circuit.

One terminal of the primary winding of the input transformer I9 is connectedto the inner conductor of the coaxial line 46 and the other terminal is connected to the outer conductor through the blocking condenser 16. one terminal of the secondary winding of the output transformer H is connected to the inner conductor of the coaxial line 41, and the other terminal is connected to the outer conductor through the blocking condenser 11.-

Except as regards the particular features of the invention which are concerned with the elements I4, l5 and 23, the amplifier will be recognised as of well-known type. The manner in which the frequency characteristic is corrected will be as described with reference to Fig. 2 and need not be again explained.

In Fig. 5 the networks [4 and [5 may take the I forms shown respectively in Figs. 3 and 4 or any other suitable forms.

It will be understood, of course, that the invention is applicable to an amplifier having any kind of gain-frequency characteristic which is not necessarily similar to curve I of Fig. 1. For

example the characteristic could be fiat, or tilted the other way and it need not be substantially straight.

It is to be noted, also, that the negative feedbackdoes produce some incomplete stabilisation against ageing at frequencies where the loop phase change is and the amount of stabilisation will be the greater as the gain around the feed-back loop is the greater. Thus when this gain is large, the amount of additional correction which must be effected by the arrangement according. to the invention Will be small,

The control grids of Similarly,

, What is claimed is:

An electric wave amplifier comprising a forward amplifying path including a plurality of thermionic valves each having an indirectly heated cathode, a power source therefor and a separate bias network coupled to said cathode,

between all of said cathodes through said bias network and said source for passing the cathode current of said valves, said thermistor being connected across at least a portion of said negative 15 feedback impedance for compensating for change in frequency characteristic due to aging of said valves.

ALLEMAN HOLLY ROCHE.

8 REFERENCES CITED The following references are of record 'in the file of this patent: I

UNITED STATES PATENTS Number Name Date 2,151,829 Barton Mar. 28, 1939 2,178,072 Fritzinger Oct. 31, 1939 2,350,951 Zinn June 6, 1944 2,367,711 Bode Jan. 23, 1945 2,428,363 Elmendorf Oct. 7, 1947 2,432,033 Nicholson, Jr. Dec. 2, 1947 2,468,082 I Chatterjea et a1. Apr.'26, 1949 i FOREIGN PATENTS Number Country Date 119,554 Australia Feb. 12, 1945 

